Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporated herein by reference in its entirety. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalyst contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. Commercial processes for the carbonylation of methanol include those described in U.S. Pat. No. 3,769,329. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.
The AO™ process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068; and EP0161874, the entireties of which are incorporated herein by reference. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate (MeAc), methyl halide, especially methyl iodide (MeI), and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at high levels, even at very low water concentrations, i.e., 4 weight percent or less, (despite the prior practice of maintaining approximately 14-15 wt. % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, e.g., 0.1 wt. %, and a specified concentration of iodide ions over and above the iodide ion that is present as hydrogen iodide. This iodide ion is a simple salt, with lithium iodide being preferred. The salt may be formed in situ, for example, by adding lithium acetate, lithium carbonate, lithium hydroxide or other lithium salts of anions compatible with the reaction medium. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, a high degree of catalyst stability and reactor productivity is achieved even when the liquid reaction medium contains water in finite concentrations as low as 0.1 wt. %. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide, which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium.
U.S. Pat. No. 5,144,068 discloses a process for producing acetic acid by reacting methanol with carbon monoxide in a liquid reaction medium containing a rhodium (Rh) catalyst and comprising water, acetic acid, methyl iodide, and methyl acetate, wherein catalyst stability is maintained in the reaction by maintaining in said reaction medium during the course of said reaction 0.1 wt. % to 14 wt. % of water together with (a) an effective amount in the range of 2 wt. % to 20 wt. % of a catalyst stabilizer selected from the group consisting of iodide salts which are soluble in said reaction medium in effective concentration at reaction temperature, (b) 5 wt. % to 20 wt. % of methyl iodide, and (c) 0.5 wt. % to 30 wt. % of methyl acetate. Suitable iodide salts may be a quaternary iodide salt or an iodide salt of a member of the group consisting of the metals of Group IA and Group IIA of the Periodic Table.
Carbonyl impurities, such as acetaldehyde, that are formed during the carbonylation of methanol may react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirable to remove multi-carbon alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to poison the catalyst used in the production of vinyl acetate, a product commonly produced from acetic acid.
Conventional techniques to remove such impurities include treating the crude acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations to yield a suitable purified acetic acid as the final product. It is also known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the purified acetic acid adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the low-water carbonylation process and purification treatment described above frequently remains somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Because a sufficient permanganate time is an important commercial test, which the acid product may be required to meet to be suitable for many uses, the presence of impurities that decrease permanganate time is objectionable. Moreover, it has not been economically or commercially feasible to remove minute quantities of these impurities from the acetic acid by distillation because some of the impurities have boiling points close to that of the acetic acid product or halogen-containing catalyst promoters, such as methyl iodide. It has thus become important to identify economically viable methods of removing impurities elsewhere in the carbonylation process without contaminating the purified acetic acid or adding unnecessary costs.
Macroreticulated or macroporous strong acid cationic exchange resin compositions are conventionally utilized to reduce iodide contamination. Suitable exchange resin compositions, e.g., the individual beads thereof, comprise both sites that are functionalized with a metal, e.g., silver, mercury or palladium, and sites that remain in the acid form. Exchange resin compositions that have little or no metal-functionality do not efficiently remove iodides and, as such, are not conventionally used to do so. Typically, metal-functionalized exchange resins are provided in a fixed bed and a stream comprising the crude acetic acid product is passed through the fixed resin bed. In the metal functionalized resin bed, the iodide contaminants contained in the crude acetic acid product are removed from the crude acid product stream.
Widely used and successful commercial processes for synthesizing acetic anhydride also involves the catalyzed carbonylation of methanol with carbon monoxide. Acetic anhydride processes have been disclosed in U.S. Pat. Nos. 5,292,948; 4,374,070; 4,115,444; and 4,046,807, the entireties of which are incorporated herein by reference.
Other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. U.S. Pat. No. 6,657,078 describes a low-water process that uses a metal-functionalized exchange resin to remove iodides. The reference also avoids the use of a heavy ends column, resulting in energy savings. U.S. Pat. No. 5,220,058 also discloses the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury. U.S. Pat. No. 5,227,524 discloses a process for removing iodide derivatives from liquid acetic acid and/or acetic anhydride comprises contacting the liquid acetic acid and/or acetic anhydride with a strong acid cation exchange resin having from about 4% to about 12% crosslinking, a surface area in the proton exchanged form of less than 10 m2 g−1 after drying from the water wet state and a surface area of greater than 10 m2 g−1 after drying from a wet state in which water has been replaced by methanol. The resin has at least one percent of its active sites converted to the silver form, preferably from 30 to 70 percent.
U.S. Pat. No. 5,801,279 discloses a method which can reduce the amount of silver or mercury dissolved in a solution after contact and can increase the usage of silver or mercury without installing new treating facilities in a process for removing iodine compounds contained in an organic medium, particularly acetic acid or a mixture of acetic acid or acetic anhydride, by contacting them with a cation exchange resin in which at least 1% of the active sites are converted to a silver form or a mercury form. This disclosed method is characterized by carrying out the operation while elevating the temperatures in stages while contacting the organic medium, particularly acetic acid or a mixture of acetic acid and acetic anhydride, containing the iodine compounds with a cation exchange resin.
U.S. Pat. No. 5,344,976 discloses that the metal ion contaminants in the acid and/or anhydride may arise from corrosion or the use of reagents in the upstream process. The patent describes the use of a cationic exchanger in the acid form to remove at least a portion of the metal ion contaminants such as iron, potassium, calcium, magnesium, and sodium from a carboxylic acid stream prior to contacting the stream with the exchanged strong acid cation exchange resin to remove C1 to C10 alkyl iodide compounds, hydrogen iodide or iodide salts.
U.S. Pat. No. 5,648,531 discloses a process for continuously producing acetic anhydride alone or acetic anhydride and acetic acid by reacting methyl acetate and/or dimethyl ether and, optionally, water and/or methanol, with carbon monoxide alone or carbon monoxide and hydrogen in the presence of a rhodium compound and methyl iodide as principal catalysts. Trace impurities causative of tar formation are distilled and separated in an evaporator and/or a subsequent refining step to remove the same. The removal of the trace impurities causative of tar formation serves to decrease the amount of tar formed as an impurity.
U.S. Pat. No. 8,759,576 discloses a process for purifying acetic anhydride. The process includes the steps of feeding a liquid crude acetic anhydride stream directly to a distillation column and separating the liquid crude acetic anhydride stream to produce a light ends stream, a sidedraw and a residue stream. The sidedraw comprises substantially pure acetic anhydride product. The distillation column is operated at a pressure less than 101 kPa. The substantially pure acetic anhydride product comprises greater than 98 wt. % acetic anhydride, has a permanganate time of greater than 10 minutes, and has an APHA color of less than 10.
While the above-described processes have been successful, the need exists for improved processes for producing acetic acid, in particular, for methods for removing acetic anhydride from those processes.